Could be simply that the wall charger isn't supplying enough to charge the Kindle (which will be simultaneously draining the battery if it's on)

I had tried to charge the Kindle with two different wall charger and the same phenomenon appeared. The led indicator light remained orange color during the charging process but the small battery icon on the top right was not shown the thunder mark which used to indicate the Kindle is receiving the electric current. After several hours of charging, the small battery icon is still white-covered only 1/5 of its space.